During the past year, we have continued to study patients with a variety of skin diseases and rheumatologic diseases for evidence of circulating antigen-antibody complexes. Patients with systemic lupus erythematosus, cutaneous vasculitis, Behcet's disease and rheumatoid arthritis have been studied utilizing methods for immune complex detection and immunofluorescence, a technique for localizing deposition of such complexes in tissues. The presence of immune complexes in all of the above conditions appears to correlate with the presence of active disease and tissue deposition. In some of these conditions we have been able to further characterize the substances as immune complexes by their molecular weight, immunoglobulin composition and in some instances by identifying responsible antigens. We will continue to isolate and purify these substances in an attempt to clarify their role in the pathogenesis of the above diseases. We have also continued to study sera from patients with bullous pemphigoid, a blistering skin disease of the elderly using complement immunofluorescence techniques. We have been extremely interested in determining the mechanisms by which the complement system is activated by these antibodies. In addition, we have also found that serum samples from bullous pemphigoid patients who do not have circulating IgG antibodies directed against the cutaneous basement membrane have complement fixing activity for the basement membrane as is seen in patients with herpes gestationis, blistering skin disease of pregnancy. These findings support our contention that these two diseases are very closely related.